To illustrate the advantages of the proposed scheme, we choose five reference schemes for benchmarking, including the NOMA scheme (labelled as “NOMA”) [1]}, the conventional OMA scheme (labelled as “OMA”) that serves as a baseline in [2]}, the virtual MIMO scheme (labelled as “Virtual MIMO”) [3]}, the hybrid MA scheme (labelled as “Hybrid MA”) in [4]}, the virtual MIMO-hybrid MA scheme (labelled as “Virtual MIMO-Hybrid MA”). Herein, the virtual MIMO-hybrid MA scheme is a modified version of the proposed scheme, which groups virtual MIMO entities into pairs to conduct NOMA and then accommodates different pairs with OMA. More specifically, we assume for the NOMA, OMA and hybrid MA schemes that each IoT device requests one data stream from the BS. With regard to both the proposed and virtual MIMO-based schemes, this amounts to the case of three data streams requested by each cluster, i.e., \(M=3\) . For fair comparison, time-division multiple access (TDMA) is considered for the OMA, virtual MIMO, hybrid MA and virtual MIMO-hybrid MA schemes to serve all the IoT devices/clusters/pairs with equal-length time slots. In addition, for both the NOMA and hybrid MA schemes, each IoT device can be treated as a special cluster with only a single IoT device. It is worthwhile to note that our analytical results also apply to the virtual MIMO-hybrid MA scheme as long as user pairing is determined. This is because each NOMA pair can be tackled independently. For illustration, an extreme pairing policy in [2]} is considered for both hybrid MA-based schemes, where the device/virtual MIMO entity having the best channel condition is paired with the one having the worst channel condition, and the rest of devices/entities are paired in the same manner.
